1. Field of Invention
The invention provides processes, reaction mixtures and two-component systems for producing silicone foam, in which hydrogen is released in a first reaction of compounds with at least one Si—H group in the presence of a basic catalyst, and the reaction mixture is crosslinked in a second reaction in the presence of a metal catalyst.
2. Description of Related Art
The use of silicone foams as sealants and adhesives is known in the art. Silicone foams are provided by numerous suppliers for various applications. In commercially available foams, the crosslinking is effected usually through an addition reaction. Such products are supplied, for example, by Sonderhoff, Wacker (Elastosil™ silicone rubber) or Dow Chemical. Corresponding foams are described, for example, in EP 0691365 B1 and WO 00/46282.
These foams have the disadvantage that the raw materials are comparatively expensive. Moreover, addition-crosslinking foams are troublesome owing to the catalysts used. Especially in applications which take place in the presence of sulfur (rubber), amines, heavy metals, etc., the problem of catalyst poisoning occurs. In this case, some or all of the silicone foam does not react to completion; the process is therefore not implementable in such applications.
A further disadvantage of the known addition-crosslinking foams is that they generally have to be heated for curing, for example in a drying oven. This firstly requires high energy expenditure; secondly, articles which are heat-sensitive or difficult to access and bulky often cannot be treated with a drying oven.
Attempts have therefore been made to provide silicone foams in which the crosslinking is effected by a condensation reaction. Such foams are produced, for example, by Compact Technologies. However, they have the disadvantage that acetic acid is released in the course of the reaction. These foams are therefore not very suitable, if at all, for the foaming of corrosion-sensitive articles, for example electronic or electric components, metallic substrates, mirrors or reflectors.
U.S. Pat. No. 2,833,732 discloses processes for producing foams based on siloxane resins. The foams are obtained by heating a reaction mixture.
EP 0 915 128 A1 relates to silicone foam materials which are obtained by a polycondensation reaction. Added physical blowing agents serve substantially for foaming.
Japanese patent JP 63-43934 discloses silicone foams which comprise a guanidiniumsilane or guanidiniumsiloxane, compounds with an Si—H group and diorganopolysiloxanes with terminal hydroxyl groups. In the reaction, the Si—H groups condense with the hydroxyl groups of the diorganopolysiloxane and release hydrogen, which foams the mixture. The reaction therefore leads to a coupled crosslinking and gas formation reaction. The two reactions do not proceed independently. In the form of the guanidinium compound, only a single catalyst is used, which constitutes a specialty reagent unusual in industry, for which there are barely any possible choices, and which simultaneously accelerates both reactions. The two reactions (gas formation and crosslinking) are therefore not controllable separately from one another, which, however, in comparable reactive systems, is indispensable for adjusting the reaction profile to the requirements of a given application. The control means for these processes and end products are therefore very limited.